Recently, as a radiographic system, a digital system using a solid-state image pickup device such as a charge-coupled device (hereinafter, referred to as an “X-ray CCD”), instead of a conventional silver-halide film (hereinafter, referred to as a film), is spreading. According to the digital system, compared with a conventional film system, there are a number of advantages: 1) an image can be observed in real time; 2) a developing device and disposal of waste liquid are not required; 3) a photo-detecting sensitivity is high, and an X-ray irradiation amount is low; 4) image processing such as enlargement and gray-scale correction is easy; 5) there is no change in a captured image with the passage of time, so that the captured image before treatment easily can be compared with that after the treatment; 6) storage space can be saved; etc.
However, when an attempt is made so as to photograph an X-ray image using such a solid-state image pickup device and using the same control method as that for ordinary photographing, it is necessary to prepare a solid-state image pickup device having an effective image pickup area with a horizontal width of 300 mm and a height of 150 mm comparable to that of a film, so that it is difficult to realize such photographing.
In order to photograph an X-ray image using a solid-state image pickup device, a photographing method has been proposed in which photographing is performed while charges in the solid-state image pickup device are transferred in accordance with the movement of an object moving in front of the solid-state image pickup device. According to this method, the solid-state image pickup device may have a width of an X-ray beam passing through a secondary slit of a radiographic system in a horizontal direction.
In a conventional radiographic system (Conventional Example 1), based on data on the previously set changing condition of a movement speed of a film during a period from the start of photographing to the end thereof at a time of photographing with a film (hereinafter, referred to as film-advance speed data), each time for the movement of a film by the width of a charge generating device constituting a solid-state image pickup device during a period from the start of photographing to the end thereof is obtained. For each time for the movement of a film during photographing, vertical transfer is performed in which charges in a vertical shift register are transferred by one stage toward a horizontal shift register in the solid-state image pickup device, and thereafter, a charge signal in the horizontal shift register is output to the outside of the solid-state image pickup device (e.g., see Patent Document 1).
Furthermore, as another conventional example (Conventional Example 2), there also is a radiographic system in which, in the same way as in the above-mentioned conventional radiographic system, a timing for transferring charges in a vertical shift register (hereinafter, referred to as vertical transfer) in a solid-state image pickup device is obtained from an angular velocity ω of a rotary arm and a functional value f(θ) responding to a rotation angle θ of the rotary arm to determine a tomographic orbit sequentially during photographing (e.g., see Patent Document 2).
FIG. 12 is a block diagram showing a configuration of a radiographic system as Conventional Example 1.
In FIG. 12, a conventional radiographic system includes an X-ray generating section 2 that generates X-rays, an X-ray CCD 203 that is a solid-state image pickup device, a sensor cassette 202 in which the X-ray CCD 203 is inserted, a cassette mounting section 201 in which the sensor cassette 202 is inserted, a rotary arm 4 that connects the X-ray generating section 2 to the cassette mounting section 201, a photographing operation managing section 204 that generates a signal to be a reference of a drive signal for controlling X-rays, a drive signal control circuit 205 that supplies a drive signal to the X-ray CCD 203 for photographing target teeth, a drive timing information output section 206 that outputs timing information for driving the X-ray CCD 203, a data processing circuit 207 that processes photographed image information, and display means 208 that displays the photographed image.
FIGS. 13A, 13B, and 13C are a front view, a right side view, and a top view, respectively, showing an outer appearance of the radiographic system of Conventional Example 1 shown in FIG. 12.
In FIG. 13, an object 1 is positioned so as to be interposed between the X-ray generating section 2 and the cassette mounting section 201 (X-ray image detecting section 3). The rotary arm 4 that connects the X-ray generating section 2 to the cassette mounting section 201 is supported rotatably by a column 209.
Next, the operation of the radiographic system configured as described above will be described.
As shown in FIG. 13C, the rotary arm 4 rotates clockwise when the periphery of the object 1 is seen from the above.
FIG. 5 is a schematic diagram schematically showing the X-ray CCD 203 (FIG. 12) during rotation. In FIG. 5, cells 114a, 114b that are charge generating devices for converting X-rays into charges are arranged two-dimensionally without any gap to constitute the X-ray CCD 203. FIG. 5 illustrates, as main components of the X-ray CCD 203, a vertical shift register 110 in which cells are connected in a horizontal direction and which transfers charges between the connected cells, and a horizontal shift register 111 in which cells are connected in a vertical direction and which transfers charges between the connected cells. Charges 113a, 113b are generated by X-rays transmitted through micro areas 112a, 112b (also referred to as 112 collectively) of the object to reach the X-ray CCD 203. In FIG. 5, A, B, . . . , H are symbols attached accessorily so as to specify a cell in a column direction, and a, b, . . . , f are symbols attached accessorily so as to specify a cell in a row direction.
As shown in FIG. 5, when the rotary arm 4 rotates, the micro areas 112 constituting the object 1 are transferred from the left to the right with respect to the X-ray CCD 203 in parallel to the vertical shift register 110.
When the micro area 112a of the object 1 is positioned as illustrated at a certain time, an X-ray transmitted through the micro area 112a reaches the cell 114a, and the charge 113a is generated in the cell 114a. Then, when the micro area 112b is transferred to the illustrated position after the elapse of a certain period, an X-ray transmitted through the micro area 112b reaches the cell 114b, and the charge 113b is generated in the cell 114b. 
Herein, when vertical transfer is performed simultaneously when the micro area 112b of the object 1 reaches the illustrated position, the charge 113a is transferred from the cell 114a to the cell 114b, with the result that the charge 113a and the charge 113b are added up in the cell 114b. 
The above-mentioned movement and addition of charges are repeated from a time when the micro area 112 reaches the front of a cell in A column d row to a time when the micro area 112 leaves the front of a cell in G column d row, and the charge 113 corresponding to the micro area 112 is transferred to a cell in H column d row in the horizontal shift register 111 and is output from the X-ray CCD 203.
In the above-mentioned process, the timing at which a charge is transferred from a cell to an adjacent cell is obtained previously by the following method.
FIG. 3 is a graph illustrating a method for obtaining a timing at which the vertical transfer of charges in the X-ray CCD 203 is performed. In FIG. 3, a curve 106 represents a temporal change of a relative movement speed of a film with respect to the cassette mounting section 201 (FIG. 12) (hereinafter, referred to as a film-advance speed) from the start of photographing to the end thereof at a time of photographing with a film.
With the width of a cell being w, a time tn (n≧1) required for a film to proceed by a distance w, 2w, 3w, . . . , nw, . . . from the start of photographing is obtained. More specifically, the time tn is obtained so that an area of a region surrounded by an x-axis, the curve 106, a straight line X=tn, and a straight line X=tn−1(n≧1) becomes w.
With t0=0, the value of a time difference Δt1, Δt2, Δt3, . . . , Δtn, . . . is obtained together with the obtained tn by the following expression.Δt1=t1−t0, Δt2=t2−t1, . . . , Δtn=tn−tn−1, . . .
Using Δtn thus obtained, a vertical transfer signal 107 is generated at a time interval illustrated in a timing chart of FIG. 4. In FIG. 4, when the vertical transfer signal 107 becomes a High level, a charge is transferred from a cell to an adjacent cell in the vertical shift register 110 (FIG. 5).
Furthermore, in the radiographic system as Conventional Example 2, during X-ray photographing, a frequency of a vertical transfer clock for a shift for controlling the movement of charges in the vertical shift register is obtained from an angular velocity ω of the rotary arm and a rotation angle θ thereof, whereby the movement of a micro area of the object is matched with the movement of a charge corresponding thereto.
Patent Document 1: Japanese Patent No. 3465582 (pages 4-5, FIGS. 1, 2)
Patent Document 2: Japanese Patent No. 3291406 (pages 3-4, FIG. 2)